In reply to goldmember:
Get something electric and powerful and you'll have it done in reasonable time. The concrete at our place was indestructible near enough - a day battering it with sledges and jumping bars did nothing more than make a few cup sized dents. We then hired a big hammer drill and modified plans to work around the concrete as much as possible.
It may of course be poor concrete for a shed - just rubble and a thin layer of concrete over the top - only one way to find out. But, if you've got borders and paths too, get something with some oomph - you'll have your work cut out just lifting it and putting it somewhere else.
Buy some really tough gloves. A long bar is invaluable for and prising the broken bits apart and getting them out of the ground.